God is love (I John 4:8). Love is part of the very being and nature of God. It is also a valid test of our sonship.
If we are united to God through faith in Christ, we share his nature and that nature is love.
A believer in Christ, who shares the nature of his father, will naturally practice love, because that is the nature of God. Our love for the brethren is proof that we are his children. Throughout the New Testament we are encouraged to love one another.
Jesus said, “By this shall all men know that you are my disciples” (John 13:35).